[ONNX] Fix typo when comparing DeviceObjType (#78085)
#77423 Introduced a typo in
https://github.com/pytorch/pytorch/blob/1db9be70a7a76a028dbeae427839c9541fd5a7ab/torch/onnx/symbolic_opset9.py#L5012-L5017
where the string `DeviceObjType` was replaced with `_C.DeviceObjType`. This PR reverts the changes to the strings.
**Tested:**
With torchvision,
```
pytest test/test_onnx.py::TestONNXExporter::test_mask_rcnn
pytest -n auto test/test_onnx.py::TestONNXExporter
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/78085
Approved by: https://github.com/datumbox, https://github.com/BowenBao, https://github.com/ezyang
